In general, semiconductor devices are used to realize system functions. The semiconductor devices each have interface circuits (input circuit, output circuit, and/or input/output circuit) and communicate with each other via the interface circuits to provide system functions. One or more of the semiconductor devices may be auxiliary device(s) detachably provided to the system.
Each of the semiconductor devices has a terminal (external terminal) for coupling with another semiconductor device. A voltage higher than a power-supply voltage in the semiconductor device, for example, ESD (electrostatic discharge) or the like, may be applied to the external terminal. The high voltage may deteriorate characteristics of elements such as transistors in the semiconductor device. Japanese Laid-Open Patent Publications No. 11-51980 and No. 2004-80346 each describe a protection circuit that is provided in interface circuits in accordance with factors for deterioration.
When no power-supply voltage is supplied to an interface circuit, a voltage higher than a power-supply voltage, for example, may be applied to the terminal (external terminal) of the interface circuit. For example, a memory card used for a digital camera is inserted into a slot of the digital camera and operated on the basis of a power-supply voltage supplied from the digital camera. In such a system, when no power-supply voltage is supplied to the interface circuit of the memory card, a voltage higher than the power-supply voltage may be applied to the external terminal of the memory card. In this case, since the protection circuit does not work effectively, the high voltage may be applied to a circuit to be protected (e.g., an input buffer) via the external terminal. This may break the circuit.